Technical Field
The field of the invention is a low noise semiconductor amplifier with switching means for externally controlling a bias voltage provided to each serial stage or to each parallel channel to compensate for extreme thermal responsivity.
Description of the Related Art
Kokubo teaches in U.S. Pat. No. 7,230,493 Bias Circuit with threshold voltage change compensation function and temperature change compensation function, a first and second temperature compensation circuit containing at least one diode. Additionally it offers a third temperature compensation circuit containing at least one diode. Each of the above temperature compensation circuits are attached to active elements i.e. transistors used for threshold voltage change compensation. At least two active elements (transistors) and two diodes are disclosed at a minimum because the invention compensates for both threshold voltage and temperature changes. This is unnecessarily complex.
Kevin Kabayashi IEEE Transactions on Microwave . . . Vol 44 No 2 Feb. 1996 teaches a monolithic DC Temperature Compensation Bias For Multistage HEMT ICs. The design incorporates a current mirror scheme. The motivation in part is to support space qualified applications which cause hybrid circuits containing discrete silicon regulators, capacitors, resistors and bond wires to be excessively costly in manufacture. Kobayashi overcomes the problems of current regulators applied to each HEMT transistor. However Kobayashi depends on a master or reference HEMT to set a current which is driven out to slave HEMTs by an HEMT op-amp. However the use of HEMT op-amps may not be cost effective and the current rather than the desired gain is kept constant.
Younsub Noh ETRI journal Volume 31 Number 3 Jun. 2009 teaches a Power Amplifier MMIC with On-Chip Active Gate Bias Circuit. Because pHEMT amplifiers are seriously affected by temperature variations, Noh discloses a three transistor active gate bias circuit. Using 3 resistors and 3 transistors, the bias circuit provides compensation of temperature variations. Increasing the drain current as the temperature increases compensates for temperature variations. Fine control over the multiple resistors and transistors affect the yield of the monolithic circuit. Many RC shunt networks are added to all gates of the amplifying stages. While the design is optimized to be monolithic, it appears to be overly complex and not cost effective.
It is difficult to accurately bias GaAs amplifier circuits for consistent performance since the same gate voltage can result in different bias currents due to large variations in Vg in production. Active feedback biasing technique is often employed which requires additional transistor and feedback mechanism. More consistent performance can be achieved by controlling the variations in the amplifier bias current using current mirroring technique. Current mirrors are known and taught for GaAs MOSFET circuits in U.S. Pat. No. 4,896,121 however no mention of temperature compensation is made.
For some applications, both a switch and at least one Low Noise Amplifier (LNA) are required. For example, a dual polarization antenna with dual ports would require two conventional monolithic LNAs and a switch to maintain the desired low noise figure. Implementing a switch for dual port selection and then a single low noise amplifier can result in degraded performance (increased noise figure) due to the switch loss. Having two LNAs and a switch increases the cost. A dual port system designer is challenged by the dilemma of a noisy single LNA solution or a costly two LNA solution.
Referring now to the figures, prior art is shown in FIGS. 1A, 1B, and 1C. One conventional system would utilize a switch 140 such as illustrated in FIG. 1A which enables either Input 1 at 141 or Input 2 at 142 to propagate to the output 149. A combination of voltages applied to resistors 147 148 cause the transistors 145 146 to be variously open or closed. The inductors 143 144 provide RF impedance matching.
Referring to FIG. 1B an antenna 110 is coupled to a circuit 120 that provides two polarized signals LHCP and RHCP. Each signal passes through one of two low noise amplifiers (LNA) 131 or 133 and thence to a switch 140. This is a costly solution because maximum utilization of the amplifying resource can only be 50% and in a handheld implementation, battery is wastefully consumed by both LNAs.
An alternative solution FIG. 1C has the circuit 120 directly coupled to the switch 140 which is followed by a single LNA 150. While economical, this design suffers from additional loss introduced by switch 140 which degrades noise figure.
What is needed is a more economical design than FIG. 1B and a less noisy design than FIG. 1C. What is more generally needed is an improved circuit for biasing GaAs transistors; an improved GaAs switch for amplifiers; and an improved GaAs Low Noise Amplifier (LNA).